Increasing the viscosity of CO2 solutions

ABSTRACT

A method of increasing the viscosity of CO 2  at least three-fold by adding (1) a viscosifying amount of a polymer having a Minimum Solubility Parameter of 6.85 or less and a plurality of electron donor atoms selected from the group consisting of O, N, and S, and (2) a sufficient amount of a cosolvent to form a one-phase solution, said cosolvent being capable of (a) dissolving at least 2% by weight of CO 2  at 25° C. and 900 psig, and (b) forming a one-phase mixture with the polymer when the mixture contains 10 weight percent cosolvent at ambient temperature and a pressure sufficient to maintain the cosolvent in the liquid phase. New compositions containing CO 2 , the defined polymer and cosolvent are also claimed as is a method for recovering oil from underground formations using a viscosified CO 2 .

This is a divisional of U.S. application Ser. No. 073,791, filed July14, 1987 now U.S. Pat. No. 4,913,235, which is a continuation-in-part ofapplication Ser. No. 058,690, filed June 3, 1987 and now abandoned,which in turn is a continuation of Ser. No. 910,041, filed Sept. 22,1986 and now abandoned, which in turn is a continuation of Ser. No.749,479, filed June 27, 1985 and now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention.

This invention relates to a method of increasing the viscosity of carbondioxide; to new compositions of matter comprising carbon dioxide and aviscosifying amount of a defined polymer-cosolvent mixture; and to amethod of recovering oil from underground subterranean formations.

In newly discovered oil fields, oil will usually be recovered by flowingfrom a producing well under the natural pressure of the fluids presentin the porous reservoir rocks. The naturally occurring pressure in theformation decreases as the fluids are removed. This is the so-calledprimary production and recovers perhaps 5% to 20% of the oil present inthe formation.

Secondary recovery methods are used to recover more of the oil, and inthese methods a fluid is injected into the reservoir to drive additionaloil out of the rocks, e.g., waterflooding. Waterflooding, of course, hasits own limitations as it is immiscible with oil and as the waterdisplaces the oil, oil remaining in the reservoir reaches a limitingvalue known as "the residual oil saturation" and oil no longer flows.There is a strong capillary action which tends to hold the oil in theinterstices of the rocks. The amount of oil recovered by secondarytechniques is usually from about 5% to 30% of the oil initially present.

In recent years, more attention has been directed to the so-calledenhanced recovery or tertiary recovery techniques. While these methodsare more expensive, they are justified by the increased price of crude.In general, these tertiary recovery methods are used to recover theresidual oil by overcoming the capillary forces which trap oil duringwaterflooding. For example, it has been suggested to add surfactants tothe flood to decrease the interfacial tension and thus allow oildroplets to move to producing wells.

Secondary recovery of oil is also possible by the miscible fluiddisplacement process. Propane, for example, would be an appropriatematerial to utilize for it is fully miscible with oil but, in general,propane is far too expensive, except in remote regions such as theArctic where it is impractical to pipe propane and thus any propaneproduced in the field could be reinjected to recover more liquidhydrocarbons.

Nevertheless, the use of crude oil miscible solvents such as propane todisplace crude oil through a formation is well known, as, for example,in the teachings of Morse in U.S. Pat. No. 3,354,953. It is alsosuggested by Morse that the viscosity of the propane can be "controlled"(i.e., increased) by the addition of kerosene. Henderson et al. teach inU.S. Pat. No. 3,330,345 to use a slug of thickened material such aspropane before flooding with an amphipathic solvent. The teachings ofDauben et al. in U.S. Pat. No. 3,570,601 relate to the recovery of oilusing viscous propane, where the propane is viscosified by firstdissolving a solid polymer such as polyisobutylene in a heavierhydrocarbon, such as heptane, and then diluting this first solution withpropane to form the oil-driving bank.

In the continental United States, carbon dioxide is generally lessexpensive. A number of carbon dioxide floods have been tried in theUnited States. The CO₂ tends to dissolve in the oil which swells with aconsequent decrease in viscosity and improvement in the flow toproducing wells. The CO₂ also extracts light hydrocarbons from the oiland this mixture of CO₂ and light hydrocarbons can in some cases reach acomposition that will miscibly displace the oil. This CO₂ -rich phasecharacteristically has a lower viscosity than the oil and tends tofinger through the formation. Early CO₂ breakthrough is of course notdesired since reservoir sweep is reduced and, also, expensive separationprocedures are required to separate and recycle the carbon dioxide. Forexample, the viscosity of carbon dioxide at usual reservoir pressuresand temperatures is on the order of a few hundredths of a centipoisewhile the oil being displaced may have a viscosity in the range of from0.1 to 100 centipoises.

It is apparent that an increase in viscosity of carbon dioxide would behelpful in decreasing the mobility of the carbon dioxide and thusincreasing the pressure gradient behind the frontal region which wouldreduce fingering and improve the reservoir sweep.

2. Descriptive of the Prior Art

The prior art describes a number of techniques to control the mobilityof carbon dioxide These techiniques are described generally in anarticle entitled "CO₂ as Solvent for Oil Recovery" by F. M. Orr, Jr. etal. (Chemtech, August 1983, page 42, et seq.). There is thewater-alternating-with-gas process where slugs of carbon dioxide areinjected alternatively with slugs of water. Also, investigations havebeen made into the use of polymers to reduce the mobility of carbondioxide. F. M. Orr, Jr. et al. report in the above article that studiesby New Mexico Petroleum Recovery Research Center indicate that onlylow-molecular weight polymers dissolve in carbon dioxide and, as aresult, only 10% to 20% increase in solution viscosity have beenobserved.

Other studies of the use of polymers for CO₂ thickening appear in"Measuring Solubility of Polymers in Dense CO₂ " by J. P. Heller et al.(Polymer Preprint, Vol. 22(2), 1981, New York ACS Meeting) andespecially "Direct Thickeners for Mobility Control of CO₂ Floods" by J.P. Heller et al. (SPE 11789, June 1983). In the latter paper, Heller etal. conclude that the search for polymeric direct thickeners has been"unsuccessful in the purpose by a wide margin." The increase inviscosity observed by Heller et al. was small and in no case greaterthan 30%, i.e., the ratio of the kinematic viscosity of the CO₂ -polymersolution to the kinematic viscosity of the CO₂ under the same conditionswas no greater than 1.3.

Recent work by J. P. Heller and J. J. Taber has been reported in"Development of Mobility Control Methods to Improve Oil Recovery by CO₂: Final Report," DOE/MC/10689-17 (available from NTIS) where the authorslist some 53 polymers which have been tried in an effort to thicken theCO₂ but with little to no success.

Work done by Heller et al. was done with pure dry CO₂ at pressure of1500 to 3160 psig and temperatures of 25 to 58° C. which would betypical of reservoirs where CO₂ flooding could be carried out. A numberof low and high molecular weight polymers were tried, and in generaltheir results showed that high molecular weight polymers were notsoluble Polymers having solubilities above one weight percent (i.e.,polybutene, polydecene and polypropylene glycol) all had molecularweights of 400 to 1000. Increasing molecular weight of the polymer ledto decreased solubility of the polymer in CO₂. Heller's work suggeststhat it is not obvious how to find polymers having a molecular weightover 1000 that have any significant solubility in CO₂. It is alsodifficult and unobvious from the teachings of the prior art on how tosubstantially increase the viscosity by dissolving very high molecularweight polymers at desirably low concentrations of the polymers. Theknown poor solvent properties of liquid and supercritical CO₂ comparedto the more usual solvents are a limiting factor when it comes todissolving large molecules such as high molecular weight polymers.

It remains, therefore, a desired objective to find a means to increasethe viscosity of carbon dioxide to achieve a viscosity of at least 0.15centipoises utilizing polymers having a molecular weight above 1000. Ithas now been found that this objective can be achieved and thatviscosity increases for the CO₂ of three-fold to 30-fold or more can beachieved utilizing certain defined cosolvents along with certain definedpolymers

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of apparatus designed for measurementof CO₂ /polymer solubility and viscosity;

FIG. 2 is a graph showing a plot of CO₂ -polymer-toluene viscosity vs.polymer concentration;

FIG. 3 is a graph showing a plot of polymer concentration vs. wt. %heptane added;

FIG. 4 is a graph showing a plot of polymer concentration vs. wt. %toluene added;

FIG. 5 is a graph showing a plot of polymer concentration vs. wt. %1-butanol added;

FIG. 6 is a graph showing a plot of CO₂ -toluene-polymer miscibilitypressure vs. temperature;

FIG. 7 is a graph showing a plot of the effect of toluene concentrationof miscibility pressure of CO₂ -toluene-polymer H solutions, and

FIG. 8 is a graph showing the effect of brine on miscibility pressure ofCO₂ -toluene-polymer H solution.

4. Description of the Invention;

In accordance with the invention, a method for increasing the viscosityof CO₂ at least three-fold has been discovered which comprises admixingwith the carbon dioxide a blend of a polymer having a molecular weightover 1000 and a cosolvent for such polymer and carbon dioxide. Thepreferred polymer is characterized as follows:

(1) it has a Minimum Solubility parameter of about 6.85 (cal/cc)^(1/2)or less; and

(2) it contains a plurality of electron donor atoms selected from theclass consisting of O, N, and S.

The cosolvent is characterized by (1) its ability to be dissolved in CO₂at 25° C. and about 950 psig to at least the two weight percent level,and (2) its ability to form a one-phase admixture with the polymer atambient temperature and a pressure sufficient to maintain the cosolventin the liquid phase and wherein the weight of cosolvent is 10% by weightof the polymer. The amount of the polymer to employ is sufficient toachieve the desired increase in viscosity for the carbon dioxide. Byutilizing the polymer - cosolvent blends of this invention, viscosityincreases of the carbon dioxide of at least three-fold can easily beachieved and viscosity increases of 20- to 3-fold or more have beenobserved. The amount of the cosolvent to employ in combination with thepolymer must be at least about 40 weight percent of the weight ofpolymer employed.

In one preferred embodiment of this invention, a blend of the polymerand the cosolvent is prepared to form a first solution. This firstsolution is then diluted with CO₂ to form an oil-driving injectableviscous CO₂ fluid which has a viscosity at least three times theviscosity of CO₂ at the desired reservoir conditions. The advantage ofthe use of the viscous CO₂ fluid is an improved sweep during floodingoperations because of improved CO₂ mobility control with a consequentreduction of fingering.

Carbon dioxide has been used as an oil recovery agent wherein recoveryis improved by taking advantage of the solubility of the carbon dioxidein the oil, causing viscosity reduction and swelling of the oil, therebyleading to increased recovery. In this use, carbon dioxide is not amiscible-type displacing agent since the pressures have been much lowerthan the minimum miscibility pressures for carbon dioxide and oil. Theviscous CO₂ mixtures of this invention can be used in any CO₂ flood ordrive process, but it is preferred that the formation pressure besufficient to at least equal the minimum miscibility pressure for thecarbon dioxide in the oil. The upper limit of pressure is the fracturepressure of the formation. The precise pressure needed in order toachieve miscibility conditions can be determined by anyone havingordinary skill in the art. Such minimum miscibility pressures aregenerally on the order of 1100 psig or more.

A typical process in which carbon dioxide is driven through thereservoir by an aqueous fluid is fully described in U.S. Pat. No.3,065,790 to Holm.

Holm et al. in the paper entitled "Mechanism of Oil Displacement byCarbon Dioxide," Journal of Petroleum Technology, December 1974, pp.1427-1438, demonstrate the advantage of maintaining carbon dioxide at apressure above the pressure required for miscible-displacement of thereservoir oil by carbon dioxide. This "miscible-displacement pressure"depends on the hydrocarbon type, formation temperature, and otherformation conditions, but is generally between about 1100 and 3000 psi.The studies of Holm et al. demonstrate that no advantage is obtained formaintaining formation pressures higher than just above themiscible-displacement pressure. At the high pressures existing inunderground formations, carbon dioxide exists as a dense fluid or liquidrather than as a gas, with the critical temperature of carbon dioxidebeing about 88° F. That is, carbon dioxide cannot be liquefied attemperatures above about 88° F. regardless of the pressures applied butcan be compressed to the state of a dense fluid. However, below 88° F.carbon dioxide exists either as a gas or a liquid depending upon thepressure applied. The typical pressures employed in enhanced oilrecovery when carbon dioxide is used are in excess of about 700 psi andthe temperatures are below about 250° F. Under these conditions thecarbon dioxide exists as a dense fluid, rather than as a gas, and if thereservoir temperature is below about 88° F. the carbon dioxide exists asa liquid.

The amount of carbon dioxide injected into the formation will, as isknown, vary for different formations and will be dependent upon totalreservoir pore volume, hydrocarbon pore volume, and other uniqueformation characteristics. In carrying out the process of thisinvention, a slug of viscous CO₂ having a reservoir pore volume ofbetween about 0.001 to about 2.0 can suitably be employed withacceptable results being obtained with from about 0.02 to about 0.35pore volume slugs. The viscous carbon dioxide dense fluid can be usedalone to displace the oil in the formation or, preferably, one or moreslugs containing 0.01 to about 1 pore volume of the carbon dioxide densefluid are driven through the formation by a drive fluid. The drive fluidmay be water, brine, carbonated water or gas sufficient in quantity todrive the viscous CO₂ through the reservoir from the injection well to aproduction well.

The term "injection pressure" as used in this specification is meant todefine the pressure at which the displacement fluids enter theformation, i.e., the pressure at a point in the well bore adjacent tothe formation. The pressure at which the viscous CO₂ fluid is injectedinto the well bore is generally greater than the pressure in theformation adjacent the well bore. In general, the injection pressuremust be sufficient to obtain miscible displacement of the oil throughthe formation and out the producing well. In general, the injectionpressure is maintained above about 1500 psi and particularly good oilrecoveries are obtained when the injection pressure is maintained aboveabout 2000 psi. The upper pressure limit is of course the fracturepressure of the formation. The preferred injection pressure is obviouslythat which achieves an economic balance between oil recovery andoperational expense (See U.S. Pat. No. 4,113,011).

CO₂ Description

The CO₂ can come from any suitable source such as those described in"Miscible Displacement" by Fred I. Stalkup, Jr. (Monograph Vol. 8, HenryL. Doherty Series, ISBN NO-89520-319-7, Society of Petroleum Engineers,1983, Chap. 8, sec. 8.4). The purity of the CO₂ is important.Substantially pure CO₂ is preferred but water saturated CO₂ isacceptable since water (or brine) is usually present in the formation.Usually, the CO₂ contains at least 95% CO₂ and preferably at least 98%CO₂, the remainder being usually light hydrocarbons. The amount ofimpurities in the CO₂ which can be tolerated is a function of the typeof oil to be displaced and the type of displacement operation. For amiscible displacement operation, the CO₂ must generally be more pure andas the viscosity of the oil to be displaced increases so should thepurity of the CO₂ These factors are discussed in the paper "Correlationof Minimum Miscibility Pressure for Impure CO₂ Streams" by H. M.Sebastian, R. S. Wenger, and T. A. Renner (SPE/DOE 12648; Paper waspresented at the SPE/DOE Fourth Symposium on Enhanced Oil Recovery heldin Tulsa, OK, Apr. 15-18, 1984). It is obviously a matter of economicsregarding the cost for purification of the CO₂ versus the benefitsderived from this purification.

Polymer Description

The polymers which are useful in preparing the new compositions of thisinvention preferably have a Minimum Solubility Parameter of about 6.85(cal/cc)^(1/2) or less; and contain a plurality of electron donor atomsselected from the class consisting of O, N, and S. Preferably, theseelectron donor atoms are a part of a donor group selected from the classconsisting of siloxane; ether; thioether; sulfone; carbonyl; ester;tertiary amine; dialkylamides; and silylether. The polymers usually havea weight average molecular weight of about 1000 to 500 thousand or more,more usually from 2000 to 400 thousand.

The polymers can be solid or liquid materials at ambient conditions solong as the polymers possess the Minimum Solubility Parameter and donorgroup characteristics as set forth below. By "liquid" is meant that thepolymer will flow at ambient conditions or have a needle penetrationvalue of no less than about 200 by ASTM test D-1321, i.e., 20millimeters. It is to be noted that some of the higher molecular weightpolymers useful in this invention such as the polysiloxanes areextremely viscous so as to resemble greases or gums and have needlepenetration values above 275.

Minimum Solubility Parameter for Polymer

The polymers should have a Minimum Solubility Parameter of about 6.85(cal/cc)^(1/2) or less.

The solubility parameters for many materials have been measured and arereported in various sources such as "Handbook of Solubility Parametersand Other Cohesion Parameters" by A. F. M. Barton, CRC Press, 1983. Asnoted by Barton, published single values of solubility parameters forpolymers are not reliable guides of solubility behavior as they are fornormal solvents. For polymers, the solubility parameter are bestspecified as a range of solubility parameter values of known solventswhich either dissolve the polymer in question or at least swell (i.e.,dissolve in) the polymer in question.

It is desired, of course, to devise a simple predictor test as to whichpolymers will successfully form a viscous solution with CO₂ in thepresence of certain defined cosolvents. ASTM test method D3132-72,described in the Barton reference, provides a spectrum of solubilityparameter values for any given polymer. The ASTM test was modified byusing mixtures of n-pentane and perfluorodimethylcyclohexane as the testsolutions and adding to a weighed amount of polymer enough of the testsolution so that the polymer was 10% by weight of the total mixture. Thesolubility parameter of the test solution was varied by changing thevolume fraction concentration of n-pentane in the test solution.Solubility parameters of mixtures can be calculated by volume fractionaveraging of the solubility parameters of the individual components asper the teachings of Barton. The test solution compositions are shown onTable I below.

                  TABLE I                                                         ______________________________________                                        COMPOSITIONS AND SOLUBILITY PARAMETER OF                                      TEST SOLUTIONS MADE BY MIXING n-PENTANE                                       AND PERFLUORODIMETHYLCYCLOHEXANE                                                         Volume   Volume Percent                                                       Percent  Perfluorodimethyl-                                                                          Solubility                                  Pure Component                                                                           Pentane  cyclohexane   Parameter.sup.(a)                           ______________________________________                                        n-pentane  100       0            7.09.sup.(b)                                           90       10            6.99                                                   85       15            6.94                                                   80       20            6.89                                                   75       25            6.85                                                   72       28            6.81                                                   70       30            6.80                                                   50       50            6.60                                                   25       75            6.36                                        Perfluorodi-                                                                              0       100           6.11.sup.(b)                                methylcyclo-                                                                  hexane                                                                        ______________________________________                                         .sup.(a) Calculated by volume fraction averaging of parameters of two pur     components except where otherwise noted.                                      .sup.(b) "Handbook of Solubility Parameters and Other Cohesion                Parameters," CRC Press, Chapter 8, Table 5.                              

A series of different polymers were added at the 10 weight percent levelto the various n-pentaneperfluorodimethylcyclohexane solutions listed inTable I above in order to determine the lowest value of the solubilityparameter of a solution in which the given polymer was soluble Thislowest value solubility parameter is defined herein as the MinimumSolubility Parameter of the polymer. As will be shown below, the MinimumSolubility Parameter for the polymers to be used in the new compositionsand process of this invention is about 6.85 (cal/cc)^(1/2) or less Inother words, if a given polymer is soluble at the 10 weight percentlevel in a mixture of n-pentane and perfluorodimethylcyclohexane whereinsaid mixture has a solubility parameter of about 6.85 or less, then suchpolymer is an acceptable polymer for use in the compositions and processof this invention provided further that such polymer satisfies the othercriteria set forth in this specification. By "about 6.85" is meant6.85±0.02 since the determined value of the solubility parameter of then-pentaneperfluorodimethylcylcohexane mixture might be off by a factorof ± 0.02. By "soluble" is meant that the components form a one-phasesolution. By "insoluble" is meant that a second phase is observed (i.e.,a second liquid phase or a solid phase).

The results of this set of runs are shown in Table II below.

                                      TABLE II                                    __________________________________________________________________________    POLYMER SOLUBILITY IN TEST SOLUTIONS                                          OF KNOWN SOLUBILITY PARAMETER                                                               Reported                                                                      Solubility                                                                          Test Solution Solubility Parameter (cal/cc).sup.1/2       Polymer       Parameter                                                                           7.09                                                                             6.99                                                                             6.94                                                                             6.89                                                                             6.85                                                                             6.81                                                                             6.80                                                                             6.60                                 __________________________________________________________________________    A Polydecene  8.2.sup.b                                                                           sol                                                                              -- -- -- sol                                                                              insol                                                                            insol                                                                            insol                                  SF-0802.sup.(a)                                                             B Polydecene  --    -- -- -- sol                                                                              insol                                                                            -- -- --                                     BSW-102.sup.(a)                                                             C Polydecene  --    -- -- -- sol                                                                              insol                                                                            -- -- --                                     418-60.sup.(a)                                                              D Polydecene  --    sol                                                                              -- -- sol                                                                              insol                                                                            -- -- --                                     430-418.sup.(a)                                                             E Polydimethyl siloxane                                                                     7.4.sup.(c)                                                                         sol                                                                              -- -- -- sol                                                                              insol                                                                            insol                                                                            insol                                  60 k cSt.sup.(c) and (d)                                                    F Polydimethyl siloxane                                                                     --    sol                                                                              -- -- sol                                                                              sol                                                                              insol                                                                            insol                                                                            --                                     100 k (c)St (GE).sup.(d)                                                    G Polydimethy siloxane                                                                      --    sol                                                                              -- -- -- sol                                                                              insol                                                                            insol                                                                            insol                                  300 k (c)St (GE).sup.(d)                                                    H Polydimethy siloxane                                                                      --    sol                                                                              -- -- -- sol                                                                              insol                                                                            -- --                                     600 k (c)St (GE).sup.(d)                                                    I Poly(ethyl vinyl ether)                                                                   7-11.5.sup.(e)                                                                      sol                                                                              -- -- sol                                                                              sol                                                                              insol                                                                            insol                                                                            insol                                  #154 (low MW).sup.(c)                                                       J Poly(ethyl vinyl ether)                                                                   7-11.5.sup.(e)                                                                      sol                                                                              sol                                                                              sol                                                                              insol                                                                            insol                                                                            insol                                                                            -- --                                     #638 (high MW).sup.(c)                                                      K Poly(isobutyl vinyl                                                                       8-11.sup.(f)                                                                        insol                                                                            -- -- insol                                                                            -- insol                                                                            -- --                                     ether) #425.sup.(c)                                                         L Poly(2-ethylhexyl                                                                         7-13.sup.(e)                                                                        sol                                                                              sol                                                                              sol                                                                              insol                                                                            insol                                                                            insol                                                                            -- insol                                  acrylate) #249.sup.(c)                                                      M Poly(propylene glycol)                                                                    --    sol                                                                              sol                                                                              insol                                                                            insol                                                                            insol                                                                            -- -- insol                                  MW = 4000.sup.(c)                                                           N Poly(ethylene glycol)                                                                     --    insol                                                                            -- -- -- -- -- -- --                                     PEG-3.sup.(g)                                                               O Poly(ethylene glycol)                                                                     --    insol                                                                            -- -- -- -- -- -- --                                     PEG-5.sup.(g)                                                               P Poly(isobutylene).sup.(c)                                                                 7.5-8.sup.(e)                                                                       sol                                                                              insol                                                                            insol                                                                            insol                                                                            -- insol                                                                            -- --                                     #040A                                                                       Q Atactic polypropylene                                                                     --    insol                                                                            -- -- -- -- -- -- --                                     (heptane soluble).sup.(h)                                                   R Poly(butadiene).sup.(c)                                                                   8.04.sup.(c)                                                                        insol                                                                            -- -- -- -- -- -- --                                   S Poly(laurylacrylate)                                                                      --    sol                                                                              sol                                                                              sol                                                                              insol                                                                            insol                                                                            -- -- --                                   T Poly(laurylmethacryl-                                                                     8.2.sup.(c)                                                                         -- sol                                                                              sol                                                                              insol                                                                            insol                                                                            -- -- --                                     ate)                                                                        U Poly(octadecylmeth-                                                                       7.8.sup.(c)                                                                         sol                                                                              sol                                                                              sol                                                                              insol                                                                            insol                                                                            -- -- --                                     acrylate)                                                                   __________________________________________________________________________     .sup.(a) Polydecenebased Synfluid produced at Gulf Research & Development     Company.                                                                      .sup.(b) Calculated by a group contribution method given in K. Brandup an     E. H. Immergut, Ed., Polymer Handbook, Wiley Interscience, NY, 1966.          .sup.(c) Scientific Polymer Products, Inc. catalog.                           .sup.(d) GE means purchased from General Electric.                            .sup.(e) "Handbook of Solubility Parameters and Other Cohesion                Parameters," CRC Press, Chapter 14, Table 1.                                  .sup.(f) Reference .sup.(e), Table 2.                                         .sup.(g) Pressure Chemical Company.                                           .sup.(h) Produced at Gulf Research & Development Company.                

Referring to Table II, it can be observed that all of the polymerstested were insoluble in test solutions having a solubility parameter of6.81 or less. It can also be observed that when the solubility parameterof the test solution was 6.85, that one of the polydecenes (i.e.,Polymer A) was soluble; all of the polydimethylsiloxane polymers (i.e.,Polymers E, F, G, and H) were i0 soluble; and Polymer I representing alow molecular weight polyethylvinylether was soluble while the remainingpolymers tested in Table II were insoluble in a test solution having asolubility parameter of 6.85.

Summarizing from the results in Table II, it can be seen that onlyPolymers A, E, F, G, H, and I were soluble in the test solution having aknown solubility parameter of 6.85 or less. As will be shown later, allof these polymers except Polymer A were useful in viscosifying CO₂ to anacceptable level. Polymer A failed as it did not contain the requisitedonor group capacity as set forth below.

Donor Group Capacity of Polymer

The polymers suitable for use in the compositions and method of thisinvention should also contain a certain amount of electron donor atomcapacity. Carbon dioxide is a weak Lewis acid [See, A. L. Myers and J.M. Prausnitz, Ind. Eng. Chem. Fund., 4,209 (1965)] and, for reasonswhich are not fully understood, it appears to be preferred that thepolymers have some strong donor functionality which can interact in somedonor-acceptor fashion with carbon dioxide. This donor capacity can beevidenced by the presence in the polymer macromolecule of a plurality ofelectron donor atoms selected from the class consisting of oxygen,nitrogen, and sulfur. Preferably, the electron donor atom is from 3 to35 weight percent of the polymer molecule, more preferably from 6.5 to30 weight percent and most preferably from 10 to 27.5 weight percent ofthe polymer molecular. Polymer molecules containing mixtures of oxygen,nitrogen, and sulfur can also be employed.

Preferably, the oxygen electron donor atom is part of a donor groupselected from the class consisting of siloxane, ether, silylether,carbonyl, and ester. The nitrogen is preferably part of a donor groupselected from the class consisting of tertiary amine and dialkylamides.The sulfur is preferably part of a donor group selected from the classconsisting of thioether and sulfone.

The preferred donor groups referred to above can be represented by thefollowing formulas: ##STR1## where R would be the same or different andis a hydrocarbyl group having from 1 to 20 carbon atoms and preferably 1to 4 carbon atoms. More preferably, the hydrocarbyl group is saturated.

The above electron donating groups can be present alone or in admixturein the backbone chain of the polymer or be present as pendant groups.

Data are presented in Table III below which illustrate the effect of thepresence of donor groups in certain polymers on the solubility of thepolymers in a CP₂ -toluene mixture. Polymers A, E, F, G, H, and I fromTable II above (the only polymers from Table II having a MinimumSolubility Parameter of 6.85 or less) were tested and the results areshown in Table III below.

In each instance, the indicated amount of polymer was added along with30 cc of toluene to a reactor which was then pressured with CO₂ toapproximately 3500 psig and 35° C. In each instance, the contents of thereactor were stirred vigorously for one hour and then the contentsallowed to settle for one-half hour before displacing the contents withwater preparatory to an analysis of the contents to determine the weightpercent polymer, as set forth hereinafter in the Experimental Worksection.

                  TABLE III                                                       ______________________________________                                                Grams of                                                                      Polymer Added                                                                             Weight Percent of Polymer                                 Polymer to Toluene-CO.sub.2                                                                       Present in the CO.sub.2 -Toluene                          ______________________________________                                        A       10.1        1.55                                                      E       30.0        12.4                                                      F       30.0        10.6                                                      G       30.0        10.8                                                      H       30.0        8.8                                                       I       60.0        19.8                                                      ______________________________________                                    

Referring to Table III, in the case of Polymers E, F, G, H, and I, allof the Polymer added was found by analysis to be actually dissolved. Inthe case of Polymer A, only about 40% of the Polymer A added was foundby analysis to be actually dissolved. As will be shown later, onlyPolymer A failed to sufficiently increase the viscosity of CO₂ to auseful degree. It should be noted that Polymer A does not contain anyelectron donor groups whereas Polymers E through H contain siloxanelinkages (or groups); and Polymer I contains an ether linkage.

The above data illustrate that the Polymers for use in the subjectinvention should not only have a Minimum Solubility Parameter of about6.85 (cal/cc)^(1/2) or less as determined by ASTM Test D3132-72,modified as described above, but should also contain a plurality ofdonor groups. Polymers B, J, L, M, 0, P, R, S, and T were also testedfor their solubility in the 90% CO₂ -10 volume % toluene mixture underthe same conditions as described above, except that Polymers 0 and Rwere run using butanol as cosolvent and Polymer P was run using heptaneas cosolvent but only a trace to less than one weight percent of thePolymer was dissolved in every instance.

It is more preferred that the polymer be a polysiloxane. Thepolysiloxanes can suitably have the ##STR2## where R and R' can be thesame or different and can be hydrogen or any hydrocarbyl having from 1to 10 carbon atoms, preferably 1 to 4 carbon atoms, and most preferably1 to 2 carbon atoms;

X can be from 100 to 7,000; preferably 1,000 to 5,000; and mostpreferably 1,500 to 4,000; and

R₁ and R₂ can be the same or different and can be selected from thegroup consisting of:

(a) any hydrocarbyl group having from 1 to 10 carbon atoms, preferably 1to 4 carbon atoms, and most preferably 1 to 2 carbon atoms, or

(b) a siloxane group.

The preferred polysiloxane is where R, R', R₁ and R₂ are all methyl.

The polysiloxanes are available commercially from such companies asGeneral Electric, Dow Corning, and Union Carbide. The polysiloxanes arenormally available in accordance with their kinematic viscosity. Asnoted from Table II above, polysiloxanes having 60,000; 100,000;300,000; and 600,000 cSt at 77° F. were tested and all were foundacceptable. (These are Polymers E, F, G, and H in Table II.) Byacceptable is meant that these polysiloxanes, when used in combinationwith the cosolvents to be described below, were effective viscosifyingagents for carbon dioxide. Suitable polysiloxanes therefore are thosehaving a kinematic viscosity of 20,000 cSt to 8,000,000 cSt at 77° F.and are liquid or have needle penetration values above 200. Oneexperimental polysiloxane had a viscosity on the order of 2.3 millioncSt and a needle penetration value of greater than 380 (380 is themaximum number for the test, the higher the number the more liquid-likeis the material being tested). Another experimental polysiloxane had aviscosity of about 7 million cSt and a needle penetration value of 292.For contrast, a polyisoprene and a polyisobutylene which were solid andrubbery had needle penetration values of 32 and 46, respectively, whilea solid polyethyleneglycol (molecular weight of 1500) had a needlepenetration value of 6 and Polymer J from Table II) the high molecularweight polyvinylethylether) had a needle penetration value of 127.

Another group of preferred polymers are the polyvinylether materialsrepresented by the formula: ##STR3## where x' is H, OH, halogen, or##STR4## and

where x" is from 20 to 3000; preferably 30 to 1000;and more preferably30 to 500 and wherein R₃ can be a hydrocarbyl group having from 1 to 10,preferably 1 to 4, most preferably 1 to 2 carbon atoms.

For reasons which are not fully understood, a high molecular weightpolyethylvinylether (i.e., where x was 625) was not suitable, whereas alow molecular weight polyvinylether was (i.e., where x was 30). Asnoted, however, it is easy to distinguish between those polyvinyletherswhich are acceptable and those which are not by determining the MinimumSolubility Parameter of the polymer as noted above, and if that MinimumSolubility Parameter is 6.85 (cal/cc)^(1/2) or less, then thepolyvinylether is acceptable. Thus, the polymers for use in thecompositions and method of this invention preferably contain both theMinimum Solubility Parameter and the donor capacity characteristics.

Cosolvent

Another key element in the subject invention is the discovery thatcertain materials defined below can serve to dissolve theabove-described Polymers to form a CO₂ -polymer-cosolvent solutionwherein the viscosity of the CO₂ is increased at least three-fold.

The cosolvent should be capable of forming a one-phase admixture withthe selected polymer at ambient temperature and a pressure sufficient tomaintain the cosolvent in the liquid phase when in the admixture thecosolvent equals 10% by weight of the polymer. By a "one-phaseadmixture" is meant the cosolvent has dissolved into the polymer and aseparate liquid phase is not observed. In addition, the cosolvent shouldbe capable of being dissolved to at least the two weight percent level,preferably four weight percent level or above, into liquid CO₂ at 25° C.and 950 psig.

It is a simple matter for one having ordinary skill in the art todetermine by a simple experiment (i) whether a given liquid cosolventwill form a one-phase admixture with a selected polymer at ambienttemperature, i.e., about 25° C. and a pressure sufficient to maintainthe cosolvent in the liquid phase using a cosolvent polymer admixturewherein the cosolvent equals 10% by weight of the polymer, and (ii)similarly whether the cosolvent will dissolved in CO₂ at 25° C. and 950psig to at least the two weight percent level. If these two simple testsare met, then the selected cosolvent will be suitable for use with theselected polymer in the compositions and method of this invention.

As a class, it would appear that many materials, including the followingmaterials, are suitable for use as cosolvents in this invention:

(a) alcohols having from 1 to 8 carbon atoms, such as methanol; ethanol;isopropyl alcohol; hexanol; cyclohexanol; etc.;

(b) aromatics having a single ring and from 6 to 10 carbon atoms such asbenzene; toluene; and the xylenes;

(c) ketones having from 3 to 10 carbon atoms such as methylethylketone;dipropylketone; methyloctylketone; and acetone;

(d) carboxylic acid esters where the carboxylic acid portion has from 2to 4 carbon atoms and the ester portion has from 1 to 10 carbon atomssuch as ethylacetate; ethylpropionate; hexylacetate; etc.; and

(e) hydrocarbons having from 2 to 20 carbon atoms such as propane;pentane; propylene; cyclohexane; isobutane; heptane; methylcyclohexane;octane; butylenes; 1-octene; or mixtures thereof including refinerystreams such as naphthas, kerosene, gas oils, gasolines, etc. Preferablythe hydrocarbons are aliphatic hydrocarbons having from 2 to 10 carbonatoms.

The preferred cosolvents for use with the preferred polysiloxanepolymers are those liquid cosolvents which not only meet the criteriaset forth above, but, in addition, have a dielectric constant of lessthan 30 at 25° C. Even more preferred cosolvents for use with thepreferred polysiloxane polymers are those which, in addition, have asolubility parameter at 25° C. of 7.0 to 12.0 (cal/cc)^(1/2).

One technique for obtaining the desired cosolvent on-site is to contactthe CO₂ in a liquid-liquid extraction apparatus with recovered crude ora fraction of such crude for a sufficient time to permit the CO₂ toextract enough light hydrocarbons to function as the cosolvent. Thedesired amount of polymer would then be added to the CO₂ -lighthydrocarbon extract to form the oil-driving material.

Amounts of CO₂ Polymer and Cosolvent

The new compositions of this invention comprise from 70 to 99.9 weightpercent carbon dioxide (usually from 80 to 99 weight percent CO₂ andpreferably 85 to 99 weight percent CO₂) and a sufficient amount of amixture of a polymer and a cosolvent, both as defined above, to effectat least a three-fold increase in the viscosity of the CO₂. Usually theweight percent polymer in the mixture is from 0.05 to 20 weight percentand more usually the amount of polymer is from 0.1 to 10 weight percent.The amount of cosolvent is at least sufficient to dissolve the desiredamount of polymer in the CO₂ and, as noted above, is at least 40% byweight of the polymer employed. This amount of cosolvent is usually from0.05 to 30 weight percent of the final mixture, more usually from 0.1 to15 weight percent.

The weight ratio of the cosolvent to polymer in the new compositions ofthis invention can be from 0.4:1 to 600:1; is preferably from 0.4:1 to15:1; and more preferably from 0.5:1 to 1.5:1.

Experimental Work

The invention will be further described with reference to the followingexperimental work.

Equipment was designed for the measurement of polymer solubility and CO₂-cosolvent-polymer solution viscosity. The system consists of a one-passflow system in which pressure is maintained by a back-pressure valve (74on FIG. 1). Polymer, CO₂, cosolvent and water are mixed in an autoclave(24 on FIG. 1) and then displaced from the autoclave 24 by the slowaddition of water and passed through a capillary in a constanttemperature oven for the indirect measurement of viscosity using adifferential pressure transducer. The polymer-cosolvent-CO₂ mixture isthen passed to a collector vessel which, after vacuum removal of allcomponents except that polymer, is weighed to determine the weight ofthe polymer contained in the known volume (weight) of solution. A moredetailed description is provided below with reference to FIG. 1.

CO₂ which enters through line 8 is pumped by pump 10 (a Ruska pump)through line 12 to valve 20. Pressure gauge 16 is installed to registerthe pressure of CO₂ from line 12 through line 14. A vent valve 18 isprovided for safety reasons. Valve 20 is a three-way valve and CO₂ maybe passed through line 22 into autoclave 24 which is provided with astirrer 26. Water can be added to autoclave 24 through line 28, pump 30and line 32.

In a typical experiment, 3 ml of water and the desired amounts ofcosolvent and polymer are added to the 300 ml autoclave 24. The purposeof adding the water is to saturate the CO₂ and 3 ml of water are morethan enough to saturate the amount of CO₂ which the autoclave holds. Theautoclave 24 is then sealed and flushed twice with 250 psig of CO₂entering through line 22 in order to remove any residual air in theautoclave 24 which is thereafter heated while CO₂ is added through line22 until the desired temperature and pressure are reached, usually 35°C. and 3500 psig. During this operation, valve 34, a two-way valve, isclosed. Valve 20 is then closed to stop any further flow of CO₂ throughline 22.

The mixture of CO₂, polymer, water and cosolvent is stirred with stirrer26 for one hour and allowed to settle without stirring for one-halfhour. During the time period this mixture is being stirred and allowedto settle, valve 34 is opened to allow flow of CO₂ through valve 36,line 38 and into a constant temperature oven 40. The CO₂ is heated tothe desired temperature (35° C.) by passage through heating coil 42. TheCO₂ then passes through line 44 and filter 46 (to remove any possiblesolids) into a capillary 50 through line 48. A pressure gauge 52 isprovided as is a bypass valve 54. A differential pressure transducer 56measures the difference in pressure across the capillary 50 and thisdifferential pressure measurement is used to establish a first CO₂baseline viscosity using the well-known Pouseille Equation which relatesviscosity to a change in differential pressure (see, N. DeNevers, "FluidMechanics," Addison-Wesley, Reading, Mass., 1970, pp. 162-8, for adiscussion and calculations involved in the Pouseille Equation).

The CO₂ then passes through line 58 and switching valve 60 eitherthrough line 66 into a collector 62 containing a tared glass tube 64 andthen through line 67 to line 68 or directly to line 68. The CO₂ thenflows through sight glass 70, line 72 and backpressure valve 74. Heptaneor other suitable material is pumped through pump 76 and line 78 toclean the backpressure valve 74. Vent valves 84 and 86 are provided asnoted along with pressure gauges 80 and 82.

After the settling period described above, the CO₂ -polymer-cosolventmixture is displaced from autoclave 24 by the addition of water at therate of 0.3 ml per minute through line 32. The displacement is effected,of course, by closing valve 34 and opening valve 36. The displaced CO₂-polymer-cosolvent mixture passes through line 38 pushing CO₂ in frontof it through the capillary 50 to provide a second CO₂ baselineviscosity used for viscosity measurements. After a stable CO₂-polymer-cosolvent flow (as evidenced by plateauing of differentialpressure across the ends of capillary tube 50) is achieved, a fractionof the mixture is collected in tube 64 to determine the amount ofpolymer dissolved in the mixture. During the displacement, observationsof the various phases present can be made by means of the sight-glass 70through which the mixture passes on its way out through back-pressurevalve 74. Carbon dioxide at atmospheric pressure flows through line 88to wet test gas meter 90 used to measure CO₂ flow. Using the Ideal GasLaw and CO₂ fluid densities at the pressure and temperature of theenvironment, CO₂ gas flow rates can be related to flow rates ofcompressed CO₂.

At the end of the viscosity data collection and solubility sampling, thesystem is switched back to pure CO₂ to flush out the polymer and toobtain a third pure CO₂ viscosity baseline The last baseline viscosityensures that no polymer is left coated on the capillary which couldreduce the capillary radius and thus cause an anomalously high viscositysince the viscosity is a function of the pressure differential acrossthe tube which is, in turn, a function of the radius of the capillary.

Solubility Determination

As noted above, a fraction of the mixture is collected in tube 64 bydisplacing a portion of the pure CO₂ which was charged to collector 62earlier The CO₂ is vented and the tared glass tube 64 removed and thecosolvent evaporated in a vacuum oven (27" of water, 60° C. for 2 days)and the weight of dissolved polymer is determined by difference inweight. The weight of CO₂ is determined by wet test gas meter 90readings at the beginning of collection when valve 60 is opened todivert flow to collector 62 and at the end of collection when valve 60is closed to divert flow away from the collector and end the collectionCorrections for the presence of cosolvent are made using the nominalconcentration based on volume added and a nominal 300 ml autoclave 24volume. Polymer solubilities are expressed as weight percent in solventbased on the formula (assuming the change of volume of mixing=0):##EQU1##

Viscosity Measurement

Viscosity measurement is accomplished by measurement of the pressuredifference (DP) due to flow of CO₂ -cosolvent-polymer solution through along capillary of small radius. The Pouseille Equation relates viscosityto DP and to flow rate in cc/min and can be used to size a capillary foran expected viscosity and DP range. In these experiments, a capillary 5'long with an internal diameter of 0.010" was used.

To calculate viscosity, the ratio of differential pressure of thepolymer solution to the differential pressure of CO₂ only from thesecond CO₂ baseline DP measurement is calculated. In the waterdisplacement of the experiment, there is a constant liquid flow ratethrough the system as long as the temperature is constant and thepressure maintained by the back-pressure valve 74 is constant.Substitution into the Pouseille Equation shows that the ratio of theviscosity of the viscous CO₂ to the viscosity of CO₂ equals the ratio ofthe change in pressure over the capillary 50 for the viscous CO₂ to thechange in pressure over the capillary 50 for CO₂, as long as the flowrate is the same for both the baseline CO₂ and the CO₂ -polymerdifferential pressure data. To get actual viscosity, the viscosity ratiois multiplied by the actual carbon dioxide viscosity under theexperimental conditions. Viscosities in this application for the CO₂-polymer-cosolvent solutions are reported as viscosity ratios (relativeviscosities).

Experiments: Experimental Conditions and Parameters

A standard set of conditions was chosen for all solubility/viscosityexperiments. Unless otherwise indicated, all runs were done at 3500 psigpressure, 35° C., and with water-saturated CO₂. Under these conditions,the viscosity of CO₂ is 0.086 centipoises, the density is 0.892 g/cc,and the water solubility is 1800 ppm_(w).

It was found that in the absence of a cosolvent the addition of water insufficient amounts to saturate the CO₂ had a deleterious affect on thesolubility of polymers in the CO₂. Nevertheless, since water is alwayspresent together with oil in underground formations, any CO₂ flood wouldbecome saturated with water while traversing the formation from theinjection to the producing well. Therefore, water was added to the CO₂-cosolvent-polymer mixtures to ensure a water-saturated system whichwould more closely approximate the conditions found in a formation.

Properties of Polymers Used

The properties of all of the polymers from Table II are set forth belowin Table IV for reference.

                                      TABLE IV                                    __________________________________________________________________________                                Kinematic          Minimum Weight %               Polymer         Molecular   Viscosity.sup.(b)  Solubility                                                                            Electron               from            Weight.sup.(a)                                                                            at 77° F. in                                                                         Donor                                                                              Parameter.sup.(g)                                                                     Donor Atom             Table II                                                                           Polymer Family                                                                           M.sub.n                                                                             M.sub.w                                                                             Centistokes                                                                           Liquid.sup.(e)                                                                      Group                                                                              (cal/cc).sup.1/2                                                                      in the                 __________________________________________________________________________                                                           Polymer                A    Polydecene   579    616                                                                              .sup. 8.sup.(c)                                                                       Liquid                                                                              None 6.85    0                      B    Polydecene   889  1,228                                                                              26.5.sup.(c)                                                                          Liquid                                                                              None 6.89    0                      C    Polydecene  1,352                                                                               2,290                                                                              88.1.sup.(c)                                                                          Liquid                                                                              None 6.89    0                      D    Polydecene --    --    --      Liquid                                                                              None 6.89    0                      E    Polydimethylsiloxane                                                                     --    110,000.sup.(d)                                                                     60,000  Liquid                                                                              Siloxane                                                                           6.85    21.6                   F    Polydimethylsiloxane                                                                     --    153,000.sup.(d)                                                                     100,000 Liquid                                                                              Siloxane                                                                           6.85    21.6                   G    Polydimethylsiloxane                                                                     --    193,000.sup.(d)                                                                     300,000 Liquid                                                                              Siloxane                                                                           6.85    21.6                   H    Polydimethylsiloxane                                                                     --    197,000.sup.(d)                                                                     600,000 Liquid                                                                              Siloxane                                                                           6.85    21.6                   I    Polyvinylethylether                                                                       1,351                                                                               2,824                                                                              4,692   Liquid                                                                              Ether                                                                              6.85    22.2                   J    Polyvinylethylether                                                                       5,746                                                                                43,512                                                                            --      Rubbery                                                                             Ether                                                                              6.94    22.2                   K    Polyvinylisobutylether                                                                   11,319                                                                               31,892                                                                             --      Solid Ether                                                                              >7.09   16.0                   L    Poly 2-ethylhexyl-                                                                       --    --    --      Liquid                                                                              Ester                                                                              6.94    9.9                         acrylate                                                                 M    Polypropyleneglycol                                                                       2,539                                                                               3,941                                                                              930     Liquid                                                                              Ether                                                                              6.99    27.6                   N    Polyethyleneglycol                                                                         591.sup.(f)                                                                          633.sup.(f)                                                                      --      Liquid                                                                              Ether                                                                              >7.09   36                     O    Polyethyleneglycol                                                                        1,464.sup.(f)                                                                       1,545.sup.(f)                                                                      --      Solid Ether                                                                              >7.09   36                     P    Polyisobutylene                                                                           9,220                                                                               24,808                                                                             --      Solid None 7.09    0                      Q    Atactic Polypropylene                                                                    NOT RUN             Solid None >7.09   0                      R    Polybutadiene                                                                            --       900.sup.(f)                                                                      --      Liquid                                                                              None >7.09   0                      S    Polyaurylacrylate                                                                         6,441                                                                               14,241                                                                             --      Liquid                                                                              Ester                                                                              6.94    13.3                   T    Polyaurylmethacrylate                                                                     4,760                                                                               23,510                                                                             --      Liquid                                                                              Ester                                                                              6.94    12.5                   U    Polyoctadecyl-                                                                            4,030                                                                               19,405                                                                             --      Solid Ester                                                                              6.94    9.5                         methacrylate                                                             __________________________________________________________________________     .sup.(a) M.sub.n and M.sub.w determined by GPC using a polydecene             calibration curve derived from known polydecene standards.                    .sup.(b) Viscosity determined by ASTM Test Method D445.                       .sup.(c) Viscosity determined at 210°  F.                              .sup.(d) M.sub.w determined by GPC with a polystyrene calibration curve.      .sup.(e) LIQUID means LIQUID at ambient conditions. By Liquid is meant a      material which flows or has a needle penetration value by ASTM Test D1321     of no less than about 200 at 77° F.                                    .sup.(f) Values reported by vendorsee Footnote .sup.(g) in Table II.          .sup.(g) Taken from Table II above.                                      

Properties of Cosolvents Used

A number of cosolvents were employed in the experiments to follow andthe properties of the cosolvents are shown in Table V below.

                                      TABLE V                                     __________________________________________________________________________    PROPERTY OF COSOLVENTS                                                                             Solubility                                                                              Solubility in                                             Dielectric                                                                              Parameter Liquid CO.sub.2 (Wt %).sup.(d)                                                           10% Swelling                                                                         10% Swelling                 Cosolvent  Constant at 25° C..sup.(a)                                                       (cal/cc).sup.1/2(c) at 25° C.                                                    at 25° C. and 950                                                                 Polymer E.sup.(f)                                                                    Polymer                      __________________________________________________________________________                                                     l.sup.(f)                     1.                                                                             Methanol 32.6      14.47     .sup.  M.sup.(e)                                                                         N      Y                             2.                                                                             Ethanol  24.3      12.71     M          Y      --                            3.                                                                             Propanol 20.1      11.88     --         Y      --                            4.                                                                             1-Butanol                                                                              17.1      11.39     --         Y      --                            5.                                                                             1-Octanol                                                                              10.3.sup.(b)                                                                            10.32     --         .sup.  YY.sup.(k)                    6.                                                                             Cyclohexanol                                                                           15.0      11.39     4          Y      --                            7.                                                                             2-Methoxyethanol                                                                       16.0.sup.(h),(i)                                                                        11.39     M          Y      Y                             8.                                                                             Ethylene Glycol                                                                        37.7      14.61       0.2      N      Y                             9.                                                                             Acetone  20.7      9.88      M          Y      Y                            10.                                                                             Methylethyketone                                                                       18.51.sup.(b)                                                                           9.29      M          Y      Y                              Tetrahydrofuran                                                                        7.61.sup.(g),(h)                                                                        9.09      --         Y      Y                              Ethyl Acetate                                                                          6.02      9.09      M          Y      Y                              Acetonitrile                                                                           36.2.sup.(i)                                                                            11.88     M          N      Y                              Toluene  2.38      8.90      M          Y      Y                              Heptane  1.92      7.38      M          Y      Y                              Propylene                                                                              1.87.sup.(b)                                                                            7.17      M          --     --                             Propane  1.61.sup.(b)                                                                            6.57      M          --     --                             Isobutane                                                                              --        6.92      --         --     --                             Hydrogen Sulfide                                                                       9.04.sup.(j)                                                                            8.44.sup.(l)                                                                            M          --     --                           __________________________________________________________________________     .sup.(a) From Handbook of Chemistry and Physics, 58th ed., CRC Press,         1977, pp. E55-8.                                                              .sup.(b) At 20° C.                                                     .sup.(c) From A. M. F. Barton, "Handbook of Solubility Parameters and         Other Cohesion Parameters," CRC Press, 1984, Chapter 8, Table II, pp.         142-149.                                                                      .sup.(d) A. W. Francis, J. Phys. Chem., 58, 1099 (1954).                      .sup.(e) "M" means miscible in all proportions.                               .sup.(f) Polymer from Table II and Y means "yes" and N means "no".            .sup.(g) See Footnote .sup.(c) above, Chapter 8, Table II, p. 172.            .sup.(h) At 30° C.                                                     .sup.(i) From A. J. Gordon and R. A. Ford, The Chemists Companion, Wiley      interscience, NY, 1972.                                                       .sup.(j) At 78° C.                                                     .sup. (k) Took longer than 48 hours to make the admixture.                    .sup.(l) Estimated from FIG. 1, p. 115 of Barton Reference in Footnote        .sup.(c) above.                                                          

Referring to Table V, the "10% Swelling" notation refers to the criteriawhether the cosolvent is capable of forming a one-phase admixture withthe selected polymer at ambient temperature, i.e., about 25° C. and apressure sufficient to maintain the cosolvent in the liquid phase (inall cases in Table V atmospheric pressure) when, in the admixture, thecosolvent equals 10% by weight of the polymer. Propylene, propane,isobutane and Hhd 2S were not run to determine if they would form theone-phase admixture as equipment was not available to liquefy thesematerials at 25° C. It can be inferred, however, that these materialswill form the one-phase admixture since each of these materials wasfound to be an effective cosolvent as seen from the results of RunNumbers 38-41 in Table II to follow.

Isobutane was not run for solubility in liquid CO₂ since it is similarto propane and heptane (cosolvents 18 and 16, respectively, in Table V)which were miscible with CO₂.

Tetrahydrofuran is a five-membered cyclic ether and was not tested forsolubility in CO₂ as it is expected to be miscible since the A. W.Francis reference in Footnote (d) indicates that diethylether,dibutylether and p-dioxane are miscible.

CO₂ Viscosity Runs

A series of experiments was run using the apparatus of FIG. 1 with eachof the Polymers from Table II except Polymers C, D, K, N, Q, and U todetermine the viscosity of the CO₂ -polymer-cosolvent mixture; theconcentration of the polymer in the mixture (i.e., wt % polymer in themixture) and the viscosity ratio of the mixture. By "viscosity ratio"(or relative viscosity) is meant the ratio of the viscosity of the CO₂-cosolvent-polymer solution to the viscosity of pure CO₂ under the sameconditions of temperature and pressure. The results are shown in TableVI below.

                                      TABLE VI                                    __________________________________________________________________________              Polymer    Cosolvent        Composition of Mixture                                                                       CO.sub.2                 Run       Wt. Added                                                                           Co-  Volume                                                                              Cosolvent                                                                           Donor                                                                              CO.sub.2                                                                          Cosolvent                                                                           Polymer                                                                            Cosolvent Polymer        Number                                                                             Polymer                                                                            (g)   solvent                                                                            Added (ml)                                                                          Swelling                                                                            Group                                                                              Wt %                                                                              Wt %  Wt % Viscosity                __________________________________________________________________________                                                         Ratio                     1   A    10.1  Toluene                                                                            30    .sup. Nonesup.(a)                                                                        90.2                                                                              8.2   1.6   1.24                     2   B    10.33 Toluene                                                                            30    Y     None 91.1                                                                              8.2   0.7   1.34                     3   C    Not Run                                                                             --   --    --    None --  --    --   --                        4   D    Not Run                                                                             --   --    --    None --  --    --   --                        5   E    40.0  Toluene                                                                            30.0  Y     Siloxane                                                                           79.5                                                                              8.1   12.4 22.9                      6   F    30.2  Toluene                                                                            30.0  Y     Siloxane                                                                           81.3                                                                              8.1   10.6 23.5                      7   G    30.0  Toluene                                                                            30.0  Y     Siloxane                                                                           81.1                                                                              8.1   10.8 34.1                      8   H    24.4  Toluene                                                                            30.0  Y     Siloxane                                                                           83.1                                                                              8.1   8.8  29.5                      9   I    50.23 Toluene                                                                            30.0  Y     Ether                                                                              70.0                                                                              10.2  19.8  4.81                    10   J    13.12 Toluene                                                                            30.3  Y     Ether                                                                              90.9                                                                              8.2   0.9  1.0                      11   K    Not Run                                                                             --   --    --    Ether                                                                              --  --    --   --                       12   L    12.75 Toluene                                                                            30.0  Y     Ester                                                                              91.7                                                                              8.1   0.2  1.0                      13   M    10.05 1-butanol                                                                          30.0  Y     Ether                                                                              92.1                                                                              7.7   0.2  1.2                      14   N    Not Run                                                                             --   --    --    --   --  --    --   --                       15   O    6.0   1-butanol                                                                          30    --    Ether                                                                              92.2                                                                              7.7   0.1  1.1                      16   P    4.56  heptane                                                                             22.95                                                                              Y     None 94.7                                                                              5.2   0.1  1.0                      17   Q    Not Run                                                                             --   --    --    None --  --    --   --                       18   R    8.9   1-butanol                                                                          30.0  Y     Olefin                                                                             91.9                                                                              7.7   0.4  1.0                      19   S    10.01 Toluene                                                                            46.2  Y     Ester                                                                              87.9                                                                              11.5  0.6  1.2                      20   T    10.02 Toluene                                                                             56.44                                                                              Y     Ester                                                                              86.4                                                                              13.4  0.2  1.0                      21   U    Not Run                                                                             --   --    --    Ester                                                                              --  --    --   --                       __________________________________________________________________________     .sup.(a) Y means "yes", i.e., the cosolvent formed a onephase admixture       with the polymer wherein the cosolvent is 10% by weight of the polymer.  

Referring to Table VI, it can be seen that only Runs 5 through 9 usingPolymers E through I resulted in at least a three-fold CO₂ viscosityincrease. These polymers represent various polysiloxanes andpolyvinylethylether. Polymer A possessed the necessary MinimumSolubility Parameter as seen from Table IV above but had no donor groupcapacity and thus resulted in little CO₂ viscosity increase even thoughan excellent cosolvent, i.e., toluene was employed. Polymer B (Run 2)showed a small increase in viscosity, i.e., about a 30% increase. Thisresult confirms the work of Heller et al. Polymers C and D were not runas the results should be redundant over Polymers A and B since thesepolymers are also polydecenes.

Polymer K was not run as its Minimum Solubility Parameter was too highas were those of Polymers N and Q. Polymer U was not run as it wasdeemed redundant over Polymer T.

Thus, the results in Table VI illustrate that certain families ofpolymers are suitable for use in the subject invention, namely, thepolysiloxanes and polyvinylethers albeit even some members of thesefamilies can be excluded if they do not possess the Minimum SolubilityParameter and/or donor characteristics as defined above.

Further experiments were performed in a manner similar to theexperiments set forth in Table VI using a variety of cosolvents in orderto determine the different types of cosolvents which can be employedwith either the polysiloxanes or the polyvinylethers The results for thepolysiloxane (Polymer E)-cosolvent runs are set forth in Table VII belowand the results for the polyvinylethylether (Polymer I) cosolvent runsare set forth in Table VIII below. Polymer E was selected asrepresentative of polysiloxanes and Polymer I as representative ofpolyethylvinylethers.

                                      TABLE VII                                   __________________________________________________________________________    POLYSILOXANE - COSOLVENT DATA                                                                                   Cosolvent                                   Wt.                   Volume Co-  Solubility                                                                          Composition    CO.sub.2 --            Polymer E             Cosolvent                                                                            solvent                                                                            Parameter                                                                           of Mixture     Cosolvent-             Run  Charged          Charged                                                                              Swell-                                                                             (cal/cc).sup.1/2                                                                    CO.sub.2                                                                          Cosolvent                                                                           Polymer                                                                            Polymer E              Number                                                                             (g)   Cosolvent  (ml)   ing.sup.(a)                                                                        at 25° C.                                                                    (wt %)                                                                            (wt %)                                                                              (wt %)                                                                             Viscosity              __________________________________________________________________________                                                           Ratio                  22    8.3  None       0      --   --    99.8                                                                              0.0    0.2 1.1                    23   29.8  Ethylene Glycol                                                                          30.0   N    14.61 89.2                                                                              10.1   0.7 1.0                    24   30.7  Methanol   30.0   N    14.47 90.1                                                                              7.4    2.5 1.9                    25   30.6  Acetonitrile                                                                             30.0   N    11.88 91.4                                                                              7.4    1.2 1.2                    26   29.9  Ethanol    30.0   Y    12.71 82.1                                                                              7.4   10.5 14.9                   27   24.5  1-Butanol  30.0   Y    11.39 81.6                                                                              7.6   10.8 20.9                   28   30.0  2-Methoxy Ethanol                                                                        30.0   Y    11.4  81.8                                                                              8.9    9.3 18.9                   29   25.7  Cyclohexanol                                                                             30.0   Y    11.39 81.1                                                                              8.9   10.0 19.3                   30   30.0  1-Propanol 30.0   Y    11.88 82.1                                                                              7.6   10.3 17.5                   31   30.7  1-Octanol  30.0   .sup.   Y.sup.(b)                                                                  10.32 81.6                                                                              7.8   10.6 17.7                   32   30.3  Methyl Ethyl Ketone                                                                      30.0   Y    9.29  81.6                                                                              7.6   10.8 20                     33   30.1  Acetone    30.0   Y    9.88  82.2                                                                              7.4   10.4 20.4                   34   29.2  Ethyl Acetate                                                                            30.0   Y    9.09  80.4                                                                              8.4   11.2 19.2                   35   30.1  Tetrahydrofuran                                                                          30.0   Y    9.09  81.2                                                                              8.3   10.5 19.8                   36   27.3  Toluene    30.0   Y    8.90  81.2                                                                              8.1   10.7 20.2                   37   29.1  Heptane    30.0   Y    7.38  84.2                                                                              6.5    9.3 18.1                   38   30.8  Propylene  67.3   --   7.17  80.6                                                                              9.63   9.7 15.53                  39   30.1  Propane    60.72  --   6.57  81.2                                                                              9.0    9.8 17.57                  40   30.8  Isobutane  74.21  --   6.92  80.6                                                                              10.7   8.7 21.50                  41   30.1  Hydrogen Sulfide                                                                         27.65  --   8.44.sup.(c)                                                                         80.64                                                                            9.6    9.7 18.30                  __________________________________________________________________________     .sup.(a) See Footnote .sup.(a) Table VI.                                      .sup.(b) This took longer than 48 hours to form the onephase admixture.       .sup.(c) See Footnote .sup.(k) of Table V.                               

Referring to Table VII, a variety of cosolvents were found acceptablewith Polymer E to result in CO₂ viscosity increases of about 10- to20-fold Methanol, ethylene glycol and acetonitrile (Run Numbers 23-25)were found unacceptable as they did not swell into the polymer to form aone-phase admixture. The experimental procedure for the determination ofwhether the cosolvent could form a one-phase admixture with the polymerwas as follows: about one gram of polymer was added to a glass vial andabout 0.1 gram of cosolvent (10% by weight of the polymer) was added atambient conditions. The mixture was allowed to stand without stirringfor at least 48 hours. Visual tests were made periodically to determineif a one-phase admixture resulted (i.e., of the cosolvent swelled intoor dissolved into the polymer).

The materials in Runs 38-41 were gaseous at atmospheric pressure andequipment was not available to liquefy the same at ambient temperature,i.e., about 25° C. Nevertheless, each of propylene, propane, isobutane,and H₂ S was admixed with CO₂ and Polymer E in the amounts shown inTable VII and very significant CO₂ viscosity increases were obtainedshowing that such materials would indeed swell or dissolve into thePolymer E if liquefied.

The results using Polymer I are set forth in Table VIII below.

                                      TABLE VIII                                  __________________________________________________________________________    POLYETHYLVINYLETHER - COSOLVENT DATA                                                                           Cosolvent                                                                     Solubility                                                                          Composition                                                Volume       Parameter                                                                           of Mixture     CO.sub.2 --Colsolven                                                          t-                      Run  Wt. Polymer I  Cosolvent                                                                            Cosolvent                                                                           (cal/cc).sup.1/2                                                                    CO.sub.2                                                                          Cosolvent                                                                           Polymer                                                                            Polymer I               Number                                                                             Charged (g)                                                                           Cosolvent                                                                            Charged (ml)                                                                         Swelling.sup.(a)                                                                    at 25° C.                                                                    (wt %)                                                                            (wt %)                                                                              (wt %)                                                                             Viscosity               __________________________________________________________________________                                                          Ratio                   42   31.8    None   0      --    --    99.5                                                                              0      0.5 1.0                     43   30.4    Methanol                                                                             30.0   Y     14.47 82.3                                                                              7.5   10.2 --.sup.(b)              44   30.1    Acetonitrile                                                                         30.0   Y     11.88 81.5                                                                              7.4   11.1 1.66                    45   30.0    Ethanol                                                                              30.0   Y     12.71 83.2                                                                              7.5    9.3 1.67                    46   55.1    1-Butanol                                                                            35.0   Y     11.4  71.4                                                                              8.7   19.9 4.31                    47   30.3    Toluene                                                                              30.0   Y     8.90  81.5                                                                              8.1   10.4 2.04                    48   50.3    Toluene                                                                              30.0   Y     8.90  77.0                                                                              8.1   14.9 3.68                    49   60.0    Toluene                                                                              40.0   Y     8.90  70.0                                                                              10.2  19.8 4.81                    50   60.2    Toluene                                                                              40.0   Y     8.90  70.4                                                                              10.2  19.4 5.04                    51   50.2    Heptane                                                                              51.2   Y     7.40  71.2                                                                              10.1  18.7 3.39                    __________________________________________________________________________     .sup.(a) See Footnote .sup.(a) on Table VI.                                   .sup.(b) Equipment failure but would expect viscosity ratio of about 1.6      since 10.23% polymer dissolved in mixture when compared to Runs 44 and 45                                                                              

Referring to Table VIII, Run 42 shows that no increase in viscosity ofthe CO₂ is achieved in the absence of a cosolvent, i.e., the viscosityratio was 1.0. Runs 43 through 52 illustrate the use of variouscosolvents with Polymer I. All of the cosolvents listed in Table VIIIwere found acceptable for use with Polymer I as they swelled (dissolved)into the polymer at the 10 weight percent level. Viscosity increases ofthreefold or greater were observed in Runs 46 and 48 through 51.Although Runs 44, 45, and 47 did not result in a viscosity ratio of atleast three, they demonstrate that the cosolvents used in these runs areacceptable. The viscosity increases as the amount of polymer increasesas seen by a comparison of Runs 47 through 50.

As noted above, the viscosity of any given CO₂ -polymer-cosolvent systemis a direct function of the amount of polymer which is dissolved intothe system. For tertiary recovery of oil (or secondary if desired), theviscosity of the CO₂ system is desirably from 0.15 centipoises to about10 centipoises which means that at least a three-fold increase in theviscosity of neat CO₂ is desired.

The data in Table IX below illustrate the increase in viscosity of a CO₂-toluene-polysiloxane system at 3500 psig and 35° C. as a function ofpolymer concentration.

                                      TABLE IX                                    __________________________________________________________________________    EFFECT OF POLYMER CONCENTRATION ON VISCOSITY                                                              Composition of Mixture                            Run  Polymer from                                                                         Wt. Polymer                                                                          Volume Toluene                                                                         CO.sub.2                                                                          Toluene                                                                             Polymer                                                                            CO.sub.2 Toluene                   Number                                                                             Table II                                                                             Charged (g)                                                                          Charged (ml)                                                                           Wt %                                                                              Wt %  Wt % Polymer Viscosity                  __________________________________________________________________________                                               Ratio                              53   F      1.43   30.0     91.34                                                                             8.18  0.48 1.30                               54   F      3.09   30.0     90.79                                                                             8.18  1.03 1.55                               55   F      6.01   30.0     89.75                                                                             8.17  2.08 2.12                               56   F      10.1   30.0     88.13                                                                             8.16  3.71 3.04                               57   F      15.0   30.0     85.38                                                                             8.14  5.48 5.61                               58   F      18.1   30.0     85.33                                                                             8.14  6.53 7.92                               59   F      24.1   30.0     83.41                                                                             8.12  8.47 14.16                              60   F      27.2   30.0     82.29                                                                             8.11  9.60 18.18                              61   F      30.2   30.0     81.27                                                                             8.10  10.63                                                                              23.53                              62   G      1.70   30.0     91.18                                                                             8.18  0.63 1.47                               63   G      3.01   30.0     90.63                                                                             8.18  1.19 1.70                               64   G      6.40   30.0     89.37                                                                             8.17  2.46 3.22                               65   G      10.50  30.0     88.03                                                                             8.16  3.82 3.95                               66   G      15.50  30.0     85.75                                                                             8.14  6.10 9.06                               67   G      20.10  30.0     84.48                                                                             8.13  7.39 16.51                              68   G      25.3   30.0     83.12                                                                             8.12  8.77 21.34                              69   G      30.0   30.0     81.08                                                                             8.10  10.81                                                                              34.14                              70   H      1.62   30.0     90.79                                                                             8.18  1.02 1.70                               71   H      3.17   30.0     90.56                                                                             8.18  1.26 1.95                               72   H      6.04   30.0     89.56                                                                             8.17  2.27 3.18                               73   H      7.94   30.0     88.53                                                                             8.16  3.30 3.95                               74   H      9.71   30.0     88.25                                                                             8.18  3.60 5.19                               75   H      14.92  30.0     86.42                                                                             8.14  5.44 10.11                              76   H      17.80  30.0     86.17                                                                             8.14  5.70 13.78                              77   H      24.41  30.0     83.08                                                                             8.12  8.80 29.48                              __________________________________________________________________________

Referring to Table IX, the viscosity ratio increases as theconcentration of dissolved polymer increases for each of the threedifferent viscosity polysiloxanes employed One skilled in the art with afew simple experiments can easily determine the concentration of polymerto employ to achieve any desired increase in viscosity for a given CO₂-polysiloxane-toluene system. Similar results would be expected usingother polymercosolvent systems. The data from Table IX are showngraphically in FIG. 2.

A series of runs was made using polysiloxane Polymer E (from Table II)and either heptane, toluene, or n-butanol as the cosolvent in CO₂ at3500 psig and 35° C. where the concentrations of polymer and cosolventwere varied. The results are shown on FIGS. 3, 4, and 5. Referring toFIG. 3, it is observed that the concentration of Polymer E substantiallyincreases when the heptane concentration is about four weight percent ormore. This is interpreted to mean that the plait point of this ternarysystem is about four weight percent heptane, about nine weight percentpolymer and the remainder CO₂ at 3500 psig and 35° C. Similar resultsare apparent for the ternary systems shown in FIGS. 4 and 5 although, ofcourse, the precise plait point compositions are different. By a plaitpoint is meant that concentration of components in a three-componentsystem where a substantially two-phase three-component system changes,sometimes abruptly, to a one-phase three-component system. (See, forexample, Physical Chemistry, Second Edition, Gilbert W. Castellan,Addison-Wesley Publishing Company (1971) Sections 15-12 and 15-13.)

It is clear from FIGS. 3, 4, and 5 that it is necessary to use asufficient amount of the cosolvent to achieve the plait pointconcentrations for any given cosolvent-polymer-CO₂ system at a fixedtemperature and pressure. It is necessary to achieve the plait pointconcentration in order to achieve a one-phase CO₂ -polymer-cosolventsystem in accordance with this invention. It is observed that theminimum weight ratio of cosolvent to polymer is about 0.4 in order toachieve the desired one-phase system. Of course, higher concentrationsof cosolvent can be used and the precise compositions will be dictatedby economics. The amount of polymer to use, as noted earlier, is thatamount which is sufficient to achieve the desired viscosity in the CO₂-polymer-cosolvent system.

Further experimental work was performed in a high-pressure sight-glassapparatus to observe the phase behavior of certain CO₂-toluene-polysiloxane compositions under varying conditions oftemperature and pressure.

The apparatus and procedures were as follows:

A Jerguson see-through sight glass contained in a constant temperaturebox and provided with a rocking mechanism for mixing its contents wasused for the solubility studies. A light source located behind the sightglass was used to illuminate its contents during visual observation.Pressure in the sight glass was adjusted by the introduction or removalof mercury contained in a hand-driven piston pump. To start a test, themercury level was lowered in the sight glass to create a vacuum ofappropriate volume and this volume next filled with CO₂. At specificconditions of temperature and pressure, the volume of CO₂ present in thesight glass could be established, and knowing the proper density, theweight of CO₂ determined. Appropriate quantities of polymers andcosolvent were introduced into the CO₂ phase through appropriate valvingusing a mechanically operated hand pump.

With all three components present in the sight glass, the temperature ofthe system was raised to the desired value and the pressure increased bythe injection of mercury into the sight glass until the three componentswould form a clear single-phase solution. The pressure of the systemwould now be slowly lowered until the minimum pressure for miscibilitywas reached and this value recorded. The proximity of the miscibilitypressure can be established by observing the behavior of the solution asthe pressure of the system is lowered. As the system approaches themiscibility condition, the polymer phase separates from the continuoussolvent phase in the form of very fine particles which refract light andgive the system a yellow coloration. A certain appropriate intensity ofcoloration was picked as a measure of miscibility. Even though thischoice appears arbitrary, it was found that increasing the pressure only50 psia resulted in a clear solution while decreasing the pressure 50psia resulted in phase separation.

In a first set of experiments, four different viscosity polysiloxanes(Polymers E, F, G, and H from Table II above) were admixed with tolueneand dry CO₂ and the mixture introduced into the sight-glass apparatusand the phase behavior studied by varying the temperature and pressure.The amounts of polymer in the mixtures were about 7 to 8 weight percentand the amount of toluene was about 7 to 8 volume percent as noted onFIG. 6 which summarizes the results. Referring to FIG. 6, each linerepresents the loci of pressures required at a given temperature toachieve a homogeneous solution. Conditions above the miscibility locilines represent conditions where one-phase behavior was observed for theparticular mixture being studied. Conditions below the miscibility locilines represent conditions where more than one-phase is observed. It wasalso noted that as the temperature increased, a higher pressure isrequired to maintain a one-phase solution. It is also unexpected that asthe viscosity of the polysiloxane is increased up to 300,000 cSt(Polymers E, F, and G), the pressure requirement to achieve a one-phasesolution is decreased. For the 600,000 cSt material (Polymer H),increased pressures are required to achieve a one-phase system. Theresults of FIG. 6 suggest that one could predict the temperatures andpressures required for a polysiloxane to form a one-phase solution usingtoluene as the cosolvent for polysiloxanes of any particular kinematicviscosity within the range studied. There are probably a family ofcurves which could easily be generated by those skilled in the art foreach polymer-cosolvent-CO₂ system including the amounts of the variouscomponents in the mixture.

A next set of experiments was run as above, except the system consistedof six weight percent of a 600,000 cSt polysiloxane (Polymer H fromTable II above); varying amounts of toluene; and the remainder CO₂. Thepurpose of the experiments was to determine if a lower pressure could betolerated to achieve a one-phase system using larger amounts of toluene.

The results of this set of experiments are shown on FIG. 7. Referring toFIG. 7, it is observed that the pressure needed to form a miscibilityloci line is reduced as the amount of toluene is increased in themixture, as expected.

From FIGS. 6 and 7, it is observed that the pressure to achieve aone-phase system can be reduced by increasing the cosolvent content or,to a lesser degree, the viscosity of the polymer. Knowing thetemperature and pressure of the reservoir, one can reasonably predictfrom data such as that in FIGS. 6 and 7 the composition of the CO₂-cosolvent-polymer mixture that will give the desired one-phasebehavior.

Yet another set of experiments was performed to determine the effect onthe one-phase system of the presence of brine. By "brine" is meant oneweight percent NaCl and 0.01 weight percent CaCl₂ in water. The resultsare shown in FIG. 8. The system studied consisted of 7.9 weight percentof a 600 cSt polysiloxane (Polymer H from Table II above); 8.6 volumepercent toluene; with the remainder being CO₂, i.e., mixture A on FIG.8. The results of these experiments are shown on FIG. 8.

Referring to FIG. 8, it was surprising to find that the use ofbrine-saturated CO₂ resulted in a lower pressure miscibility loci lineindicating that one-phase operations could be achieved at lowerpressures in the presence of brine. The saturation of the CO₂ usingdistilled water resulted in a similar degree of pressure reduction.

It is apparent from the above that the viscosity of CO₂ can be increasedmore than three-fold by adding to the CO₂ a viscosifying amount of apolymer so long as a cosolvent is also employed. The new compositionscomprising the CO₂, polymer and cosolvent can be used, for example, todeposit the polymer as a thin film on a surface, but such compositionsare particularly useful for the recovery of oil from an undergroundoil-bearing formation.

Obviously, many modifications and variations of the invention, ashereinabove set forth, can be made without departing from the spirit andscope thereof, and therefore only such limitations should be imposed asare indicated in the appended claims.

What is claimed is:
 1. A solution comprising CO₂, a polymer, and acosolvent; wherein said polymer has a Minimum Solubility Parameter ofabout 6.85 (cal/cc)^(1/2) or less; wherein in said solution the weightpercent of CO₂ is at least 70; and the amount of cosolvent is at leastsufficient to dissolve said polymer in said CO₂.
 2. A solutioncomprising CO₂, a polymer, and a cosolvent; wherein, in said solution,the weight percent of CO₂ is at least 70; the weight percent of saidpolymer is at least sufficient to increase the viscosity of said CO₂ bya factor of at least 3; and wherein the amount of cosolvent is at leastsufficient to dissolve said polymer in said CO₂.
 3. A solution inaccordance with claim 2 wherein the weight ratio of said cosolvent tosaid polymer is at least 0.4 and wherein said polymer has a MinimumSolubility Parameter of about 6.85 (cal/cc)^(1/2) or less and contains aplurality of a donor atom selected from the class consisting of O, N,and S; and wherein said cosolvent will dissolve at least 2 weightpercent CO₂ at 25° C. and about 950 psig.
 4. A composition in accordancewith claim 3 wherein said cosolvent and said polymer form a one-phasemixture at 10 weight percent cosolvent at ambient temperature and apressure sufficient to maintain the cosolvents in the liquid phase.
 5. Acomposition in accordance with claim 2 wherein said polymer is a liquidpolymer and wherein the weight percent of CO₂ is from 70 to 99.9; theweight percent of said polymer is from 0.05 to 20; and the weightpercent of said cosolvent is from 0.05 to
 30. 6. A composition inaccordance with claim 5 wherein the polymer has a plurality of donorgroups selected from the class consisting of siloxane; carbonyl;tertiary amine; dialkylamides ether; silylether; thioethers; sulfone;and ester.
 7. A composition in accordance with claim 6 wherein thepolymer is selected from the class consisting of:(i) a polysiloxanehaving the formula: ##STR5## where R and R' can be the same or differentand can be hydrogen or any hydrocarbyl having from 1 to 10 carbonatoms;x can be from 100 to 7,000; and R₁ and R₂ can be the same ordifferent and can be selected from the group consisting of:(a) anyhydrocarbyl group having from 1 to 10 carbon atoms; and (b) a siloxanegroup; and (ii) a polyvinylether having the formula: ##STR6## where x'is H, OH, halogen, or ##STR7## and where x" is from 20 to 3,000 and R₃is a hydrocarbyl group having from 1 to 10 carbon atoms.
 8. Thecomposition in accordance with claim 7 wherein the polymer is apolysiloxane having the formula: ##STR8## where R₁ ; R₂ ; and R' and xare as defined above.
 9. The composition according to claim 8 wherein R,R₁, R₂, and R' are hydrocarbyl groups having from 1 to 2 carbon atomsand x is from 1,500 to 4,000 and the polymer has a kinematic viscosityof from 40,000 to 8,000,000 cSt at 77° F.
 10. The composition accordingto claim 9 wherein the cosolvent is selected from one or more from theclass consisting of alcohols having from 1 to 8 carbon atoms; singlering aromatics having from 6 to 10 carbon atoms; ketones having from 3to 10 carbon atoms; carboxylic acid where the carboxylic acid esterportion has from 2 to 4 carbon atoms and the ester portion has from 1 to10 carbon atoms; and hydrocarbons having from 2 to 20 carbon atoms. 11.The solution according to claim 10 wherein the cosolvent has adielectric constant of less than 30 at 25° C. and a solubility parameterat 25° C. of 7.0 to 12.0 (cal/cc)^(1/2).
 12. The solution according toclaim 11 wherein the amount of CO₂ is from 80 to 99 weight percent; from0.1 to 10 weight percent of the polysiloxane; and from 0.1 to 15 weightpercent of the cosolvent.
 13. The solution according to claim 7 whereinthe polymer is a polyvinylether having the formula: ##STR9## where x' isH, OH, halogen, or ##STR10## and where x" is from 20 to 3000 and R₃ is ahydrocarbyl group having from 1 to 10 carbon atoms.
 14. The solutionaccording to claim 13 wherein R₃ has from 1 to 2 carbon atoms and x' isfrom 30 to
 500. 15. The solution according to claim 14 wherein thecosolvent is selected from one or more from the class consisting ofalcohols having from 1 to 8 carbon atoms; single ring aromatics havingfrom 6 to 10 carbon atoms; ketones having from 3 to 10 carbon atoms;carboxylic acid esters where the carboxylic acid portion has from 2 to 4carbon atoms and the ester portion has from 1 to 10 carbon atoms; andhydrocarbons having from 2 to 20 carbon atoms.
 16. The solutionaccording to claim 15 wherein the amount of CO₂ is from 80 to 99 weightpercent; from 0.1 to 10 weight percent of the polyvinylether; and from0.1 to 15 weight percent of the cosolvent.
 17. A method of increasingthe viscosity of CO₂ by at least three-fold which comprises adding tosaid CO₂ (1) a viscosifying amount of a polymer having a MinimumSolubility Parameter of about 6.85 (cal/cc)^(1/2) or less and aplurality of electron donor atoms selected from the class consisting ofO, S, and N, and (2) a sufficient amount of a cosolvent to form aone-phase system, said cosolvent being liquid or a compressible gas andcapable of (i) dissolving at a concentration of at least 2% by weight inliquid CO₂ at 25° C. and about 950 psig and (ii) forming a one-phasemixture with the polymer at ambient temperature and a pressuresufficient to maintain the cosolvent in the liquid phase when saidmixture contains 10 weight percent of said cosolvent.
 18. The methodaccording to claim 17 wherein the polymer is a liquid polymer selectedfrom the class consisting of:(i) a polysiloxane having the formula:##STR11## where R and R' can be the same or different and can behydrogen or any hydrocarbyl having from 1 to carbon atoms;x can be from100 to 7,000; and R₁ and R₂ can be the same or different and can beselected from the group consisting of:(a) any hydrocarbyl group havingfrom 1 to 10 carbon atoms; and (b) a siloxane group; and (ii) apolyvinylether having the formula: ##STR12## where x' is H, OH, halogen,or andwhere x" is from 20 to 3,000 and R₃ is a hydrocarbyl group havingfrom 1 to 10 carbon atoms and wherein the cosolvent is selected from oneor more from the class consisting of alcohols having from 1 to 8 carbonatoms; single ring aromatics having from 6 to 10 carbon atoms; ketoneshaving from 3 to 10 carbon atoms; carboxylic acid esters where thecarboxylic acid portion has from 2 to 4 carbon atoms and the esterportion has from 1 to 10 carbon atoms; and hydrocarbons having from 2 to20 carbon atoms.
 19. The method according to claim 18 wherein thepolymer is a polysiloxane having the formula: ##STR13## where R, R₁, R₂,and R' are hydrocarbyl groups having from 1 to 2 carbon atoms and x isfrom 1,500 to 4,000 and the polymer has a kinematic viscosity of 40,000cSt to 8,000,000 cSt at 77° F. and the cosolvent has a dielectricconstant of less than 30 at 25° C. and a solubility parameter at 25° C.of 7.0 to 12.0 (cal/cc)^(1/2).
 20. The method according to claim 18wherein the polymer is a polyvinyl ether having the formula: ##STR14##where x' is H, OH, halogen, or ##STR15## and where x" is from 20 to 3000and R₃ is a hydrocarbyl group having from 1 to 10 carbon atoms.